I don't know how much younger your sister is, so this advice might be way off. We are skipping Space on our upcoming trip due to the single file seating and the darkness. our 5 yr old is more than tall enough, but I know it will be too much for him. I was considering giving my boys (5 & 2.5) a glow bracelet a day to wear so they had their only little bit of light to focus on if a ride got too dark or spooky for them.